fail to create on databases that aren't UTF8

[eeeebbbbrrrr]

PL/Rust can't support databases that aren't UTF8 because of Rust's requirements around Strings being proper UTF8. This PR causes CREATE EXTENSION plrust to raise an ERROR in this case:

[v14.6][2495941] plrust=# create extension plrust;
ERROR:  PL/Rust only supports UTF8-encoded databases

This might be a little heavy-handed now but it gives us time to consider other options for relaxing this requirement.

The decision here stems from the fact that, currently, pgx blindly assumes all TEXT Datum conversions are valid UTF8, when they might not be. That's a problem too, but it's still not clear the RightWay to resolve that and likely wouldn't do us any good here in plrust anyways.

[workingjubilee]

Reviewed the out-of-band discussion and I think this is good.